The world's growing energy needs require the use of various carbonaceous materials, especially those that have previously been considered less suitable for use as fuel. Many carbonaceous materials are less suitable for use as fuel because their moisture contents are very high. Combusting carbonaceous materials with high moisture contents imposes a penalty on heating efficiency, because of the additional energy required to vaporize the moisture during combustion, and because of the additional equipment costs necessary to handle the additional water vapor. High moisture content also necessitates non-optimal operating conditions in combustion facilities to avoid the formation of corrosive sulfuric acid in flue gas. Moist carbonaceous materials are also susceptible to freezing in cold climates, increasing the difficulty of handling and transportation. In addition, high moisture content increases transportation costs, because when moving an amount of carbonaceous material representing a certain heating value, any moisture retained in the material adds weight but not heating value.
As but one example of the general problem of high moisture in carbonaceous materials, consider the extensive deposits of subbituminous coal in the western United States. This coal is not widely relied upon in the eastern United States, despite its useful low sulfur content, because the western subbituminous coal retains significant amounts of moisture, ranging from 20 percent to 50 percent. The high moisture content of western subbituminous coal contributes significantly to the overall cost of the coal to eastern purchasers, because transportation costs represent the majority of the cost of western coal to an eastern coal purchaser. Furthermore, existing eastern coal consumers are inhibited from switching to western coal because the eastern combustion equipment often cannot be economically adapted to accommodate the moist western coal. Different aspects of the same general problem arise in the combustion and other uses of bituminous coals, lignite, peat, biomass, and other carbonaceous materials. Accordingly, there is a history of attempts to upgrade carbonaceous materials by lowering the moisture content of the materials.
Attempts to lower the moisture content of carbonaceous materials can be traced back at least to U.S. Pat. No. 1,679,078 to Fleissner. More recent derivatives of the Fleissner process are represented, for example, in U.S. Pat. No. 4,514,912 to Janusch. Fleissner described the treatment of brown coal, also known as lignite, with steam at certain temperatures and pressures. Fleissner describes placing the lignite in an autoclave, and driving off moisture from the lignite by heating it with steam under superatmospheric pressure, and then depressurizing the autoclave. The carbonaceous material product of the Fleissner process, while having a lowered moisture content at first, is hygroscopic, and will rapidly become moist once exposed to atmospheric moisture. The product of the Fleissner product also tends to create dust and be susceptible to spontaneous combustion, two significant problems in coal handling. Furthermore, the Fleissner process is not economically attractive due to low energy efficiency.
Accordingly, it is desirable to provide energy-efficient methods, systems, and apparatuses for upgrading carbonaceous materials. It is also desirable to provide carbonaceous materials that not only have decreased moisture and impurity contents, but also have stability to atmospheric moisture, decreased dustiness, and reduced likelihood of spontaneous combustion.